334 CHAPTER 8. MULTICHANNEL SYSTEMS
later increased to 32, resulting in a 160-Gb/s transmission over 9300 km [18]. In a
2001 experiment, a 2.4-Tb/s WDM signal (120 channels, each operating at 20 Gb/s)
was transmitted over 6200 km, resulting in aNBLproduct of almost 15 (Pb/s)-km (see
Table 8.1). This should be compared with the first fiber-optic cable laid across the
Atlantic ocean (TAT-8); it operated at 0.27 Gb/s withNBL≈ 1 .5 (Tb/s)-km. The use of
WDM had improved the capacity of undersea systems by a factor of 10,000 by 2001.
On the commercial side, WDM systems with a capacity of 40 Gb/s (16 channels at
2.5 Gb/s or 4 channels at 10 Gb/s) were available in 1996. The 16-channel system cov-
ered a wavelength range of about 12 nm in the 1.55-μm region with a channel spacing
of 0.8 nm. WDM fiber links operating at 160 Gb/s (16 channels at 10 Gb/s) appeared
in 1998. By 2001, WDM systems with a capacity of 1.6 Tb/s (realized by multiplexing
160 channels, each operating at 10 Gb/s) were available. Moreover, systems with a 6.4-
Tb/s capacity were in the development stage (160 channels at 40 Gb/s). This should be
contrasted with the 10-Gb/s capacity of the third-generation systems available before
the advent of the WDM technique. The use of WDM had improved the capacity of
commercial terrestrial systems by a factor of more than 6000 by 2001.
8.1.2 Wide-Area and Metro-Area Networks
Optical networks, as discussed in Section 5.1, are used to connect a large group of
users spread over a geographical area. They can be classified as a local-area network
(LAN), metropolitan-area network (MAN), or a wide-area network (WAN) depending
on the area they cover [6]–[11]. All three types of networks can benefit from the WDM
technology. They can be designed using the hub, ring, or star topology. A ring topology
is most practical for MANs and WANs, while the star topology is commonly used for
LANs. At the LAN level, a broadcast star is used to combine multiple channels. At the
next level, several LANs are connected to a MAN by using passive wavelength routing.
At the highest level, several MANs connect to a WAN whose nodes are interconnected
in a mesh topology. At the WAN level, the network makes extensive use of switches
and wavelength-shifting devices so that it is dynamically configurable.
Consider first a WAN covering a wide area (e.g., a country). Historically, telecom-
munication and computer networks (such as the Internet) occupying the entire U.S. ge-
ographical region have used a hub topology shown schematically in Fig. 8.3. Such net-
works are often called mesh networks [19]. Hubs or nodes located in large metropoli-
tan areas house electronic switches, which connect any two nodes either by creating
a “virtual circuit” between them or by usingpacket switchingthrough protocols such
as TCP/IP (transmission control protocol/Internet protocol) andasynchronous transfer
mode(ATM). With the advent of WDM during the 1990s, the nodes were connected
through point-to-point WDM links, but the switching was being done electronically
even in 2001. Such transport networks are termed “opaque” networks because they
require optical-to-electronic conversion. As a result, neither the bit rate nor the modu-
lation format can be changed without changing the switching equipment.
An all-optical network in which a WDM signal can pass through multiple nodes
(possibly modified by adding or dropping certain channels) is called optically “trans-
parent.” Transparent WDM networks are desirable as they do not require demultiplex-
ing and optical-to-electronic conversion of all WDM channels. As a result, they are